1. Field of the Invention
The present invention relates to the field of local area network communications, and in particular, to line drivers in local area networks.
2. Related Art
A local-area network ("LAN") is a communication system that enables personal computers, work stations, file servers, repeaters, data terminal equipment ("DTE"), and other such information processing equipment located within a limited geographical area such as an office, a building, or a cluster of buildings to electronically transfer information among one another. Each piece of information processing equipment in the LAN communicates with other information processing equipment in the LAN by following a fixed protocol (or standard) which defines the network operation. Information processing equipment made by different suppliers can thus be readily incorporated into the LAN.
The ISO Open Systems Interconnection Basic Reference Model defines a seven-layer model for data communication in a LAN. The lowest layer in the model is the physical layer which consists of modules that specify (a) the physical media which interconnects the network nodes and over which data is to be electronically transmitted, (b) the manner in which the network nodes interface to the physical transmission media, (c) the process for transferring data over the physical media, and (d) the protocol of the data stream.
IEEE Standard 802.3, Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications, is one of the most widely used standards for the physical layer. Commonly referred to as Ethernet, IEEE Standard 802.3 deals with transferring data over twisted-pair cables or co-axial cables. The 10 Base-T protocol of IEEE Standard 802.3 prescribes a rate of 10 megabits/second ("Mbps") for transferring data over twisted-pair cables.
The constant need to transfer more information faster, accompanied by increases in data processing capability, necessitated an expansion to data transfer rates considerably higher than the 10-Mbps rate prescribed by the 10 Base-T protocol. As a consequence, a protocol referred to as 100 Base-T was developed for extending IEEE Standard 802.3 to accommodate data moving at an effective transfer rate of 100 Mbps through twisted-pair cables. Under the 100 Base-T protocol, certain control bits are incorporated into the data before it is placed on a twisted-pair cable. The result is that the data and control signals actually move through a twisted-pair cable at 125 Mbps.
In expanding IEEE Standard 802.3 to the 100 Base-T protocol, there are various situations in which it is desirable that the transmitter be capable of using one driver to transmit data at both the 100 Base-T rate and the lower 10 Base-T rate. Accordingly, is it preferable to use a line driver capable of driving both 10 Base-T and 100 Base-T signaling.
In particular, one set of information processing equipment should be capable of driving data moving at the 10 Mbps ("Meg") rate or the 100 Meg rate without having to make any adjustments when the data transfer rate changes from 10 Meg to 100 Meg and vice versa.
FIG. 1 illustrates the data transmit path 100 of communication in the LAN operating in 100 Base-T. During data transmission, a communication unit operating on the LAN, such as a computer 117, generates a data signal T1 which is converted into differential form for transmission on the twisted pair cable 103. For 10 Base-T transmission, this data signal T1 is Manchester coded 101 to reduce electromagnetic interference and to produce square wave pulses. These pulses are then filtered 101 such that the square wave pulses are basically sinusoidal waves. These waves then go through a waveshaping filter to generate filtered differential data signals T1+/-.
In this description a pair of differential signals means two signals whose current waveforms are out of phase with one another. The individual signals of a pair of differential signals are indicated by reference symbols respectively ending with "+" and "-" notation--e.g., S+ and S-. The composite notation "+/-" is employed to indicate both differential signals using a single reference symbol--e.g., S+/-.
For 100 Meg transmission, scramble and filter 119 scrambles data signal T1 and converts data signal T1 to differential format. Encoder 121 MLT-3 codes the data signal to generate trinary differential signals T2+/-. A 10 Meg amplifier signal driver 107 and a 100 Meg amplifier signal driver 109 take these differential signals T1+/- and T2+/-, respectively, and generate voltage-sourced differential signals T10+/- and T20+/- respectively, to drive a primary load 105 and to transmit them on twisted pair cable 103.
Transformer 111 has a primary winding 111A and a secondary winding 111B which isolate the twisted-pair cable 103 from the circuitry producing the transmit signals. Primary winding 111A terminates at a primary load 105 and secondary winding 111B terminates at a secondary load 113. Secondary load 113 couples to a connecting unit 115, which couples to twisted-pair cable 103. Primary winding 111A couples to a resistive load 105. It is across this resistive load 105 that either sine wave 10 Base-T signaling or MLT-3 100 Base-T signaling must be created.
Line drivers can handle either transfer rates of 10 Meg and 100 Meg, or both 10 Meg and 100 Meg. However, all of these line drivers have the disadvantage that often the differential output voltage falls outside a specified tolerance range, causing the line driver circuit to be ineffective. Such inaccuracy may result due to some of the components of the line driver circuits have excess circuit variations, such as circuit errors, component errors, transformer errors, and inaccurate load resistor values.
Thus, a need exists to correct the differential output voltage occurring in both 10 Meg and 100 Meg line drivers when the differential output voltage falls outside the specified tolerance range.